Why Is the Pulmonary Circulation Reduced in the Human Fetus?

According to the American Heart Association, fetal circulation does not include pulmonary circulation because the fetus is unable to breathe oxygen in the womb. Instead, the mother's placenta oxygenates the blood for the fetus.

Fetal circulation is a lot different than the circulation the baby will have once it is born, states the American Heart Association. The mother's placenta accepts any oxygen-poor blood from the baby that leave the fetus through the umbilical arteries located in the umbilical cord. The blood is diffused through the placenta and picks up oxygen. Next, the oxygenated blood can re-enter the fetus through the umbilical vein that is also located in the umbilical cord. The oxygen-rich blood from the umbilical vein passes through the fetal liver and then enters the right side of the fetal heart. Next, the oxygen-rich blood flows through the patent foramen ovale and goes from the right atrium to the left atrium, and then it can go to the left ventricle and leave the heart through the aorta. Oxygen-poor blood also enters the right atrium and is shunted to the right ventricle through the ductus arteriosus. This allows the oxygen-poor blood to reach the lower half of the fetal body and leave the umbilical artery to become oxygenated by the placenta. Once the baby is born, the patent foramen ovale and the ductus arteriosus should close and allow pulmonary circulation to begin.